1. Technical Field
The embodiments herein generally relate to communication networks, and more particularly, to ranging in communication systems.
2. Description of the Related Art
Ranging refers to a process of estimating and adjusting the delays, transmits power and frequency offsets of various customer premise equipment (CPEs) with respect to one or more references fixed at a base station (BS). The process of ranging is indispensable in systems using orthogonal frequency division multiple access (OFDMA) that require supporting CPEs that are positioned at large distances from the BS causing a delay larger than the delay corresponding to a cyclic prefix (CP) used. The process of ranging enables the BS to command a CPE to advance a timing boundary sufficiently so that a received frame boundary of the CPE approximately aligns with the frame boundary of other CPEs that are already linked to the BS. The process of ranging also enables power and frequency corrections to be applied to the CPEs so as to facilitate all the CPEs to appear co-located and synchronized.
In various communication standards, the process of ranging may be performed through an initial ranging and subsequently through periodic ranging. During the initial ranging, a CPE registers or re-registers with a BS. On successful completion of initial ranging, the CPE gets attached to the BS and subsequently correction and tracking of delay, power and/or frequency offset may be done through periodic ranging. The periodic ranging is done to periodically adjust various physical parameters of the CPE and to verify that the CPE is still communicatively coupled to the BS. In several exemplary scenarios initial ranging as well as periodic ranging processes may be initiated by the CPEs after the BS allots a ranging window on an uplink communication (from the CPE to the BS) that is exclusively for ranging purposes. A ranging window consists of a set of ranging subcarriers in a given set of orthogonal frequency division multiplexing (OFDM) symbols. The CPEs that need to range need to contend in the allotted ranging window. In some exemplary scenarios, in order to reduce chances of two or more CPEs ranging in a common ranging window, a random back-off algorithm may be employed.
The CPEs may also use random code division multiple access (CDMA) codes that may be used to separate ranging signals from different CPEs. Typically performance of ranging with respect to delay estimation and complexity depends on a pattern of selection of the ranging subcarriers. The ranging sub-carrier selection should preferably span the full bandwidth, should enable an unambiguous delay covering a maximum range possible, and should also enable a low complexity detection of a ranging code such as a CDMA code transmitted from a CPE to the BS. Typical ranging sub-carrier allocation techniques use randomly spaced ranging subcarriers (for example, in WiMAX standards), uniformly spaced ranging subcarriers (for example in IEEE 802.22 standards) for ranging, or allocation based on some permutation (like the output of an interleaver).
Ranging performed using uniformly spaced subcarriers imposes certain constraints on delay estimation that is performed using frequency domain correlation techniques. For instance, in IEEE 802.22 standard, a uniform spacing of 10 between the subcarriers is mentioned that causes a repetition of 10 peaks in an output of the frequency domain correlation process that leads to ambiguity in delay estimation. In randomly spaced subcarriers, the spacing between any two ranging subcarriers do not have uniformity (but are fixed once chosen). Ranging performed using randomly spaced subcarriers leads to higher complexity, as a frequency domain correlation involves multiplying received ranging subcarriers by the corresponding transmitted data on the subcarriers, and computing an inverse fast Fourier transform (IFFT). Additionally, in the case of randomly spaced subcarriers the IFFT length will be equal to the total number of subcarriers in the OFDM symbol since the relative positions of the ranging subcarriers cannot be altered. Moreover, allocation based on interleaving might not choose the spacing and number of uniformly spaced subcarriers optimally.